1. Field of the Invention
The invention relates to a controlled atmosphere system for a refrigerated container and specifically to a system for regulating the amount of nitrogen, oxygen, and carbon dioxide inside a refrigerated container resulting in an atmospheric composition that extends post harvest shelf life/or quality of fruits and vegetables.
2. Description of the Prior Art
Controlled atmosphere containers for the post-harvest transport and storage of agricultural commodities are well known in the art. Such systems are usually used in conjunction with a refrigeration unit designed to lower the temperature of the interior of the container to a desired optimum temperature for the load being shipped therein. It has been established that temperature is the most important factor in controlling the rate of deterioration of fruits and vegetables. A controlled atmosphere system is considered as a supplement to proper temperature and humidity control.
The purpose of a controlled atmosphere system is to control the amount of oxygen and carbon dioxide inside the refrigerated container to change the rate of ripening of the produce stored in the container. The system controls the amount of oxygen (O.sub.2) and carbon dioxide (CO.sub.2) by replacing them with nitrogen (N.sub.2).
In early examples of controlled atmosphere systems for refrigerated containers, the modified atmosphere within the container was established, after loading, and not further modified during the period of storage or transportation. Problems with such systems included leakage both into and out of the container which changed the atmosphere. Subsequent technologies have provided for monitoring of oxygen and carbon dioxide levels within the container and have provided nitrogen and carbon dioxide sources to allow modification of the atmosphere during storage and transportation. A drawback to such systems was that supplies of carbon dioxide and nitrogen gas were required to maintain the desired atmosphere during normal lengths of transportation. Such gas sources were bulky and heavy and were not economically feasible to provide the gases required in a typical commercial application.
Systems have been developed wherein extremely high purity nitrogen gas is generated using an air separator of a type which incorporates membranes to divide a stream of ambient air, under pressure, into its principal constituents of oxygen and nitrogen. Some of such systems utilize an electronic controller to electrically operate a valve which will selectively increase or decrease the amount of nitrogen delivered from the membrane separator to the refrigerated container.
The controller is used to operate the controlled atmosphere system in conjunction with various functions of the refrigeration system. In one such system a thermostat placed in the container is used to determine if the temperature control range of the refrigeration system exceeds the operating temperature range of the controlled atmosphere system. This information is used to determine if the controlled atmosphere system should operate. In addition the controlled atmosphere system should not operate during the defrost cycle of the refrigeration system or when the refrigeration system is cooling the container cargo to its set point. To determine these conditions, in such system the controlled atmosphere controller receives various signals from sensors, relays and other components of the refrigeration system. If the determined conditions are met, the controller allows the controlled atmosphere system to start. If during operation these signals are not received the controller suspends operation of the controlled atmosphere system.
Such systems, it has been found, do not always reliably prevent interference with the operation of the refrigeration system by the controlled atmosphere system.